1. Field of the Invention
The present invention relates to a tire pressure indicator for indicating the pressure in a tire and more particularly to a remote tire pressure indicator having a bellows which is fluidly coupled to the tire in order to provide a mechanical and visually direct reading of the pressure in the tire and which is electromagnetically coupled to a ferrite rod in order to provide an electrical signal which is related to the pressure in the tire.
2. Description of the Prior Art
U.S. Pat. No. 4,334,428, entitled Apparatus for Indicating Parameters Sensed on a Rotatable Wheel, issued to Raoul G. Fima and Jacques E. Mahieux on June 15, 1982, teaches a circuit which is mounted on a rotatable wheel of a vehicle and which varies its resonant frequency in accordance with tire pressure. U.S. Pat. No. 4,283,707, entitled Aircraft Low Pressure Tire Warning System Having Comparator Circuit for Each Axle Pair of a Four Wheel Bogie Configuration, issued to Royce F. Church on Aug. 11, 1981, teaches pressures transducers which coupling transformers electrically couple to a signal processor.
U.S. Pat. No. 4,389,884, entitled Remote Tire Pressure, issued to Gianluigi Agulia on June 28, 1983, teaches a remote system for indicating tire pressure for use in combination with a pressure transducer which is disposed inside a pneumateic tire. The pneumatic tire is mounted on a wheel rim. The remote system includes a ferrite rod which is mechanically coupled to the pressure transducer and which is disposed within the pneumatic tire. The remote system also includes a passive circuit including a first coil which is mechanically coupled to the wheel rim, a second coil which is electromagnetically coupled to the ferrite rod, which is slidably coupled to the second coil and a capacitor, all of which are electrically coupled in series to form a tuned circuit which resonates at a particular frequency which is dependent on the position of the ferrite rod, and a detector which detects the change in frequency at which the first circuit resonates thereby providing a measure of movement of the ferrite rod in response to the change in pressure of the pneumatic tire. The detector includes an oscillating circuit having a coil of several turns on a rod of highly permeable material which is disposed adjacent to the first coil of the passive circuit on the wheel rim whereby the passive circuit receives energy from the oscillating circuit. As a result of the linear movement of the ferrite rod there will be a change in the frequency at which the passive circuit resonates thereby producing a voltage across the coil of the oscillating circuit which is linearly proportional to the change of pressure in the pneumatic tire. The detector also includes an amplifier which amplifies the voltage which is electrically coupled to the oscillating circuit and a metering display which displays the amplified voltage.
U.S. Pat. No. 4,373,385, entitled Fluid Pressure Detection, issued to Raymond Boutteville and Thierry Febvret on Feb. 15, 1983, teaches a pressure sensor for measuring the pressure in an aircraft wheel tire which includes a stationary housing and a shaft connected to the wheel. A unit slidably connected to the shaft has a surface cooperating with a pair of bellows coaxial to the shaft to define a pressure chamber. A passage in the shaft communicates the pressure chamber and the tire.
U.S. Pat. No. 4,348,896, entitled Apparatus for Indicating Pressure in Pneumatic Tires, issued to Athanase Sarides on Sept. 14, 1982, teaches an apparatus for indicating pressure in a pneumatic tire on a wheel while in motion on a vehicle which includes an actuator secured to the wheel so as to rotate therewith. The acutuator has an extendible bag member in communication with air in the tire, aspring controlling extension of the bag by air in the tire to cause the bag member to be extended by an amount indicative of the air pressure in the tire, and a magnet positioned in accordance with the amount of extension of the bag member and therefore in accordance with the air pressure in the tire. A series of magnetically actuated switches are mounted on the vehicle for selective actuation by the magnet in accordance with the position of the magnet as it rotates with the tire, and indicators connected to the magnetically actuated switches indicate air pressure in the tire by indicating the magnetically actuated switch selectively actuated.
U.S. Pat. No. 4,226,126, entitled Mechanical Displacement-Electrical Signal Transducer, issued to Werner Herdon on Oct. 7, 1980, teaches in order to measure small displacement of a membrane under change of pneumatic pressure applied at opposite sides thereto, a ferromagnetic wire which is stretched between a fixed support and the membrane. An exciter coil, connected to a source of alternating current of suitable strength cyclically reversely magnetizes the ferromagnetic wire. A pick-up coil is eletromagnetically coupled to the wire to sense the change of magnetization thereof which will be in the form of sharp needle pulses as the magnetization of the wire changes, the pulse amplitude and pulse width being highly dependent on the strain in the wire and hence on the deflection of the membrane. The wire may be stressed under tension.
U.S. Pat. No. 4,250,759, entitled Digital Readout Gauge, issued to Otto Z. Vago and Frank S. Irlinger on Feb. 17, 1981, teaches a digital readout gauge which may be a tire pressure gauge which is mounted in a small portable housing. A pressure chamber is formed within the housing with a valve core deflator mounted within the housing to depress the core of a standard tire valve and permit the air pressure within the tire to be introduced into the chamber. A wall of the chamber is movable with changes of pressure and a piezoelectric crystal transducer or strain gauge transducer is mounted to be stressed in accordance with movement of this wall to develop an analog voltage in accordance with the air pressure. An electrical circuit is contained within the housing and energizable by a battery carried within the housing. A power switch is actuated to an on condition by movement of the gauge onto the tire valve to energize the electrical circuit with the analog voltage passed to an analog-to-digital converter and then to a digital readout display to display the digital value of the air pressure within the tire.
U.S. Pat. No. 4,195,531, entitled Pressure Detector Using an Average Circuit, issued to Kenji Okamura on Apr. 1, 1980, teaches a pressure detector which includes a spring-loaded diaphragm mounted in a housing to define a pressure chamber to which test fluid is introduced. A pulse generating element is provided to generate an electrical pulse in response to a displacement of the diaphragm when the fluid pressure in the pressure chamber exceeds a predetermined level. An averaging circuit is connected to the pulse generating element to convert the pulses into a signal having a voltage level representative of the mean value of the pressure variation.